This application claims priority from British Patent application number 9818885.7 filed Aug. 28, 1998 which is herein incorporated by reference.
The present invention relates to a current sensing noise thermometer.
It has long been desirable in the field of low temperature physics to find an easy way to measure precisely temperatures below the Kelvin range and into an achievable milli-Kelvin range. A known principle for such low temperature thermometry is the measurement of the thermal (Johnson) noise in a resistor, which can then be used to determine the temperature of the resistor.
One method which uses this principle involves the use of a Josephson junction shunted by a current biased resistor. The resulting Josephson frequency oscillation is influenced by the thermal noise in the resistor, causing frequency fluctuations, the variance of which is proportional to the temperature. The measurement of frequency variance, however, means that long measurement times are necessary in order to achieve good precision. In addition, the current bias causes dissipation which limits the effectiveness of the method for cooling down the resistor at sub-mK temperatures.
An alternative method is current sensing noise thermometry, in which the thermal noise currents in a resistor are measured directly using a low noise amplifier, such as a SQUID. This method can be used to achieve much faster measurements with considerable precision, and because the resistor need not be current biased, lower temperatures can be achieved in principle.
In principle, such a current sensing noise thermometer can be used as a primary thermometer (absolute thermometer) in the case where the sensor resistance, circuit time constant, gain of the SQUID read-out system, etc., have been precisely measured. This measurement could be a rather difficult task and those parameters could be changed by a different SQUID read-out system or setup from place to place or time to time. This is not a convenient way at all for using it practically. Another approach is using the current sensing noise thermometer as secondary thermometer with a known temperature point, but this still presents the problem that the calibration procedure against a known temperature is quite complicated. Typical calibration methods involve placing the resistor in liquid helium, and measuring the vapor pressure of the helium, to determine the temperature for precise calibration, or using fixed-point devices. Numerous complicated measurements and setups may also be involved in the use of known current sensing noise thermometers.
Furthermore, it is difficult to cool the resistor to temperatures sufficiently low to enable the thermometer to be used at temperatures in the mK and RK range, because of Kapitze resistance between the thermometer and substrate in known designs.
The present invention, from one aspect, provides a current sensing noise thermometer comprising a sensor resistor and a low noise amplifier for measuring the noise current in the resistor, characterized in that a coil made from superconducting material is connected in series between the resistor and the amplifier whereby to allow the superconducting transition temperature of the superconducting material to be used to calibrate the thermometer.
Another aspect of the invention provides a current sensing noise thermometer comprising a sensor resistor and a low noise amplifier for measuring the noise current in the resistor, characterized in that a superconducting coil assembly is provided, the coil assembly comprising a coil made from superconducting material and at least one superconductor positioned in close proximity to the coil whereby to influence the action of the coil, and the coil assembly is connected in series between the resistor and the amplifier whereby to allow the superconducting transition temperature of at least one superconductor of the coil assembly to be used to calibrate the thermometer.
Preferably, at least one superconductor of the coil assembly is positioned at least partially inside the coil. In a further preferred embodiment, at least one superconductor is, in use, placed in contact with an object whose temperature is to be measured.
From a further aspect the invention provides a current sensing noise thermometer comprising a sensor resistor and a low noise amplifier for measuring the noise current in the sensor resistor, characterized in that the resistor is grounded.
The advantage of using an grounded sensor resistor is that the resistor can be cooled down to the lowest possible electronic temperature, in the low xcexcK range.
Another aspect of the invention provides a method of current sensing noise thermometry comprising the steps of positioning a sensor resistor in close proximity to an object whose temperature is to be measured, measuring the noise current in the resistor by means of a low noise amplifier, and recording and/or displaying information derived from the measured noise current which represents the temperature of the resistor, characterized in that a superconducting element is connected to the sensor resistor and at least one superconducting transition temperature of the superconducting element is used for calibration purposes.
Preferably, the influence of a change in magnetic flux in the superconducting element at the superconducting transition temperature is used for calibration purposes. A plurality of superconductors may be used as the superconducting element.
In a preferred embodiment of the invention, an grounded sensor resistor is used in conjunction with a superconducting coil, or a coil assembly comprising one or more superconductors. This provides the advantage that the change in inductance of the coil at the known superconducting transition temperature of the superconductor(s) can be used to calibrate temperature measurements derived from the noise current measurements, as the resistor is cooled to low temperatures.
The low noise amplifier may be a SQUID, preferably a DC SQUID. In a preferred embodiment, the method can be used with any setup or SQUID read-out system without knowing their parameters and it does not need any external fixed temperature point. This kind of thermometer could be calibrated by itself in situ.
The coil and/or at least one superconductor preferably comprises a niobium-based superconductor.
In an alternative embodiment, a high temperature superconductor may be used for the coil and/or at least one superconductor in the coil assembly, instead of a low temperature superconducting material. By this method, a temperature measurement range from 77K to sub-mK temperatures may be achieved. In another alternative embodiment, several superconductors may be provided in the coil assembly, thereby providing several calibration temperatures, preferably over a large temperature range.
An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings in which: